Molecular transitions recently discovered at redshift z_abs_=2.059 towards the bright background quasar J2123-0050 are analysed to limit cosmological variation in the proton-to-electron mass ratio, {mu}=mp/me. Observed with the Keck telescope, the optical echelle spectrum has the highest resolving power and largest number (86) of H_2_ transitions in such analyses so far. Also, (seven) HD transitions are used for the first time to constrain {mu}-variation.